1. Field of the Invention
The invention relates in general to a method and a testing system for measuring contact resistance. More particularly, the invention relates to a method and a testing system used for measuring contact resistance of pins of an integrated circuit.
2. Description of the Related Art
In the conventional method for measuring a contact resistance, for example, an open/short circuit method, a voltage of a device is measure while applying the device with a direct current. In FIG. 1, the circuit diagram for a conventional method to determine whether the contact is good or poor is illustrated. In the circuit, a current flows into an integrated circuit chip 100 from a voltage input source 120. The testing device 150 (a probe) extracts a certain amount of the current from a signal pin 110, while the voltage difference is measured across the signal pin 110 and the voltage input source 120. Using the voltage input source 120 as a reference standard, that is, assuming the voltage input source 120 is at a level of 0 volt, if the signal pin 110 has a voltage level between -0.3 volt to -1.5 volt, the signal pin 110 is determined to be a good contact. Otherwise, beyond this range, the signal pin 110 has a bad contact.
As shown in FIG. 1, the voltage difference across the signal pin 110 and the voltage input source 120 is equivalent to a voltage across a diode 130 of about 0.7 volt. If the voltage input source 120 has a voltage level of 0 volt, the pin signal 110 then has a voltage of -0.7 volt that falls within the normal operating range (-0.3 to -1.5 volt).
Typically, the amount of the extracted current is as small as about 100 micro-amperes (.mu.A). Thus, even if the signal pin 110 is in bad contact with the probe 150 to form a non-Ohmic contact, that is, there is a voltage drop across the contact, or when the contact resistance is high, the open/short circuit test might still pass. As a result, the integrated circuit fails in the function test.
For example, while the extracted current is 100 micro-amperes, and the contact resistance is 900 Ohms (.OMEGA.), the voltage drop is only 0.09 volt. The variation is about 12.86%. Plus the original voltage difference of 0.7 volt, the total voltage difference is 0.79 which is still within the tolerable range. Therefore, the signal pin with a contact resistance as high as 900 Ohms passes the open/short circuit test. However, this high contact resistance may cause error results while performing the function test.
As described above, the conventional circuit or method to measure the contact resistance has a very low sensitivity according to the contact resistance. Therefore, even with a bad contact of the pin, it can still pass the open/short circuit test but fails in the function test to cause an error.